Make a subpackage for Makie support

[lorenzoh]

FastMakie can be a subpackage living in the FastAI repo that implements the Makie ShowBackend. Subpackages can then depend on MakieCore.jl to define recipes and we don't need Requires anymore.

Since this will be breaking as well, it should be released together with the other breaking changes #196 and #240. #224 also needs to be fixed for this.

[lorenzoh]

Closed by #240